This invention relates generally to computed tomography (CT) imaging and more particularly to methods and apparatus for generating CT imaging data using a multi-slice imaging system.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reducing scan time, helical scanning provides other advantages such as better use of injected contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
The x-ray beam is projected from the x-ray source through a pre-patient collimator that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes x-ray-absorbing material with an aperture therein for restricting the x-ray beam. In at least one known CT imaging system, a scanning mode and corresponding reconstruction method are implemented for 3:1 and 6:1 helical pitches. The 6:1 helical pitch mode is referred to as a xe2x80x9chigh speedxe2x80x9d mode because volume coverage is large, and scanning is faster along z-axis than in the 3:1 helical pitch mode. However, the scanning and reconstruction techniques used for this high speed mode have not been found suitable for scanning at greater helical pitches, for example, 8:1 or higher. One of several reasons that these techniques have not been found suitable is that the 6:1 high speed mode uses conjugate sampling pairs that are, in general, no longer valid at pitches of 8:1 or more.
For explaining the problems of the known high speed mode, it will be helpful to define a number of variables and their relationship to the geometry of a CT imaging system. Let xcex2k, k=1, . . . , 4 represent projection angles at which detector rows k intersect a plane of reconstruction. Also, let xcex2kxe2x88x92, k=1, . . . , 4 represent projection angles of conjugate samples for xcex2k that are xcfx80 earlier, so that xcex2kxe2x88x92=xcex2kxe2x88x92xcfx80xe2x88x922xcex3. Similarly, let xcex2k+ represent projection angles of conjugate samples for xcex2k that are xcfx80 later so that xcex2k+=xcex2k+xcfx80xe2x88x922xcex3.
In fan beam geometry, the detector angle, xcex3, is defined as an angle formed by any ray with respect to an isoray 50, as illustrated in FIG. 4. More particularly, xcex3m=max(|xcex3|) represents a maximum fan angle. Referring to FIGS. 5 and 6, the four adjacent graphs 52, 54, 56, 58 represent the four adjacent detector rows of one known CT imaging system. Graph 52 represents the weighting region for detector row 1. Graphs 54, 56, and 58 are for detector rows 2, 3, and 4, respectively. The labeled regions in the graphs (R1, R2, . . . , R4) are regions in which weighting functions are applied to the projection samples. Outside these regions, all weights are equal to zero. Therefore, projection data outside these regions is not needed.
In each graph 52, 54, 56, 58, horizontal axis 60 represents the detector angle, xcex3, and vertical axis 62 represents the projection angle, xcex2. Therefore, samples corresponding to a fan beam at a particular view angle are represented by horizontal lines in the graphs. Referring to FIG. 5, a lower boundary for region R1 (corresponding to detector row 1) represents conjugate samples of xcex23. Therefore, the boundary is defined by xcex23xe2x88x92.
As shown in FIG. 5, in high speed acquisition at 6:1, an iso-ray of xcex23xe2x88x92 intersects detector row 1 when row 1 is one detector-row-width away from xcex21, where xcex21 is a projection angle at which row 1 crosses a plane of reconstruction. For a 6:1 helical pitch, a table of the CT imaging system travels six times a thickness of a detector in a gantry rotation of 2xcfx80. Therefore, it takes 2xcfx80/6=xcfx80/3 to travel a single detector thickness. (In FIG. 5, xcfx80/3 thickness is 1 division of the vertical axis.) The angular span for R2, R3 and R4 is xcfx80/3, and corresponds to a detector thickness. The lower right region defined by xcex23xe2x88x92 of R1 (detector row 1) is nearly 2xcfx80/3 away from xcex21, or almost twice a detector thickness away from a point at which detector row 1 intersects the plane of reconstruction. Thus, samples acquired far away from a true sample location are used to estimate an ideal sample, which adversely affects the accuracy of the estimation. This same problem also applies for regions R2xe2x80x2 (for detector rows 2 and 4) and R1 for detector row 3. FIG. 6 represents a corresponding high-speed mode weighting pattern for 8:1 helical reconstruction, showing that the problem becomes even worse at this higher pitch.
Furthermore, the known 6:1 high speed mode reconstruction relies upon the existence of certain conjugate samples. In particular, and referring to FIG. 5 samples from rows 2 and 4 are used to perform interpolations in high speed mode, as are samples from rows 3 and 1. However, this mode is not suitable for scanning at helical pitches of 8:1 or higher. Referring to FIG. 6, it is clear that at these higher pitches, xcex24xe2x88x92 and xcex21 lines for row 1 intersect. Similarly, lines for xcex21+ and xcex24 for row 4 intersect. Because lines xcex24xe2x88x92 and xcex21+ should carry a weight of zero and xcex21 and xcex24, should carry a weight of 1, the weights for the intersecting points cannot be determined.
Another reason that the known high speed mode has not been found suitable for 8:1 and higher pitches is that an imaging system employing a 6:1 helical pitch is configured so that xcex21, xcex22, xcex23, and xcex24 are spaced xcfx80/3 apart, while 2xcex3m is slightly less than xcfx80/3. When an 8:1 helical pitch is employed, xcex21, xcex22, xcex23, and xcex24 are spaced xcfx80/4 apart. This latter configuration no longer confines conjugate regions to one side of a location at which the detector crosses the POR, as shown by regions R1 in FIG. 6. A discontinuity is created in the weighting function at the boundaries of the two regions.
It is thus seen that the known high speed imaging mode is not suitable for imaging of objects at 8:1 pitch and higher. It would therefore be desirable to provide methods and apparatus that can overcome this limitation.
There is therefore provided, in one embodiment of the present invention, a method for generating an image of an object using a multislice computed tomography imaging system. The method includes steps of: helically scanning an object with a multislice computed tomography imaging system to acquire projection data; determining a set of conjugate samples of the projection data that formulate a set of parallel projections; and reconstructing a set of images of the object using the conjugate samples.
By determining a set of conjugate samples that formulate a set of parallel projections, embodiments of the present invention make possible reconstruction of images from projection data scanned at pitches greater than 6:1, for example, 8:1 or higher.